The heat transfer performance of a tube having surface enhancements is known by those skilled in the art to be superior to a plain walled tube. Surface enhancements have been applied to both internal and external tube surfaces, including ribs, fins, coatings, and inserts, and the like. All enhancement designs attempt to increase the heat transfer surface area of the tube. Most designs also attempt to encourage turbulence in the fluid flowing through or over the tube in order to promote fluid mixing and break up the boundary layer at the surface of the tube.
A large percentage of air conditioning and refrigeration, as well as engine cooling, heat exchangers are of the plate fin and tube type. In such heat exchangers, the tubes are externally enhanced by use of plate fins affixed to the exterior of the tubes. The heat exchanger tubes also frequently have internal heat transfer enhancements in the form of modifications to the interior surface of the tube.
In a significant proportion of the total length of the tubing in a typical plate fin and tube air conditioning and refrigeration heat exchanger, the refrigerant exists in both liquid and vapor states. Below certain flow rates and because of the variation in density, the liquid refrigerant flows along the bottom of the tube and the vaporous refrigerant flows along the top. Heat transfer performance of the tube is improved if there is improved intermixing between the fluids in the two states, e.g., by promoting drainage of liquid from the upper region of the tube in a condensing application or encouraging liquid to flow up the tube in a wall by capillary action in evaporating application.
It is also desirable that the same type of tubing be used in all of the heat exchangers of a system. Accordingly, the heat transfer tube must perform satisfactorily in both condensing and evaporating applications.
In order to reduce the manufacturing costs of the heat exchangers, it is also desirable to reduce the weight of the heat transfer tube while maintaining performance.
Accordingly, what is needed is a heat transfer tube that provides suitable performance for both condensing and evaporating applications and that offers practical and economical features to end users.